Wide range radiation monitor

ABSTRACT

A unique diode sensor adapted for responding with pre-established sensitivity over a range of relatively low frequencies. A diode is connected to a dipole antenna having both conductive and resistive portions. Lumped impedances are connected in parallel with the diode and the values of these impedances are selected in coordination with the characteristics of the diode and antenna to control the sensitivity of the sensor as a function of the frequency of the illuminating energy.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to radiation monitors and more particularly, tomeasuring instruments suitable for monitoring radiation of electricfields within a frequency range extending to the vicinity of 100 GHz.

2. Description of the Prior Art

The essential components of instruments capable of monitoring radiationover a range of frequencies, are the electrical field sensitiveelements. Their characteristics, structure, and interconnection areselected to achieve a sensitivity to the field that is relatively flatover the frequency ranges of interest. Thermocouples and semiconductordiodes are common sensing elements. It has been recognized that eachtype of element, while having inherent advantages and limitations, alsoexhibits frequency limitations. Thus for example, diode sensors havebeen shown to operate most effectively at lower frequencies, e.g. below1.5 GHz, while thermocouple sensors operate most effectively at higherfrequencies, e.g. above 300 MHz.

The characteristics of various sensors also have shown that diodes aremost effective when applied in conjunction with the high reactance ofelectrically short dipole antennas. In contrast, the best sensitivity ofthermocouple elements is achieved when low resistivity materials areemployed in a thin-film configuration in conjunction with low reactancedipoles. There are other recognized characteristics which lead tocorresponding limitations. At higher frequencies the reactance of diodesensors may be dominated by shunt capacitance effects which severelylimit their high frequency response. At the other extreme, thermocouplesensors are limited in their application to the low frequency region dueto difficulties in developing thermocouples of sufficiently high filmresistivity to operate in conjunction with the higher reactance dipolesand with sufficient sensitivity to provide adequate signal levels.

One form of effective field measuring equipment is shown in theinventor's U.S. Pat. Nos. 3,641,439 and 3,794,914, issued on Feb. 9,1972 and Feb. 26, 1974, respectively. These patents disclose anear-field radiation monitor utilizing thin-film thermocouplespositioned in quadrature to measure relatively high frequency electricfields.

As further described in the inventor's article Broad-Band IsotropicElectromagnetic Radiation Monitor, published in the November 1972I.E.E.E. Transactions on Instrumentation and Measurement, one mayutilize sensors of the general type described mounted along threemutually orthogonal axes in order to achieve isotropic performance. Onthe other hand, to date, no suitable equipment is known to be availablethat is capable of operating over the broad frequency range contemplatedherein.

SUMMARY OF THE INVENTION

The present invention uses thin-film thermocouple sensors and diodesensors mounted on a single probe for monitoring an ultra-broadband offrequencies. Within the lower frequency range, the diode sensors furnishthe monitoring capability. Within the upper frequency range, thin-filmthermocouple sensors furnish the monitoring capability. The varioussensors are mounted within a single housing, yet they are maintainedessentially independent in order to prevent interaction. The outputs ofthe sensors are summed to provide a true indication of power densityover the complete wide range of frequency being monitored.

An object of the present invention is to provide a portableultra-broadband radiation detector.

Another object of the invention is to provide a portable radiationdetector having an isotropic response.

Yet another object of the invention is to provide a portable radiationdetector having pre-established sensitivity to impinging radiationthroughout both low and high frequency regions.

Still another object of the invention is to provide a portable radiationdetector utilizing both diode and thin-film sensing elements.

From one aspect, the invention resides in a unique diode sensor adaptedfor responding with pre-established sensitivity over a range ofrelatively low frequencies. From another aspect, the invention residesin the utilization of separate sensors of diverse type, for respondingover a broad range of frequencies, which exceeds the operatingcapabilities of either type of sensor, if used alone.

In the first embodiment, the energy sensing means includes a diodeconnected to a dipole antenna having both conductive and resistiveportions. Lumped impedances are connected in parallel with the diode andthe values of these impedances are selected in coordination with thecharacteristics of the diode and antenna to control the sensitivity ofthe sensing means as a function of the frequency of the illuminatingenergy.

In the second embodiment, the aforedescribed diode sensing means isarranged along a common axis with one or more thermocouple dipoles,though separated therefrom. In this embodiment, the thermocouplecomponent or components form a sensing means designed to respond withconstant sensitivity to illuminating energy over the highersubstantially adjacent frequency range which exceeds the controllableresponse of the diode sensing means.

The invention will be more thoroughly understood and appreciated fromthe following description which is made in conjunction with thedrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a pictorial illustration of a probe embodying the inventionand demonstrating the preferred orientation of the sensor elements usedtherein;

FIG. 2 is a graph plotting relative sensitivity as a function offrequency, of a probe embodying the features of the invention, thefrequency scale being logarithmically presented;

FIG. 3 is an enlarged schematic illustration of a set of sensorelements, aligned and arranged in accordance with a preferred embodimentof the invention.

FIGS. 4A, 4B are schematic representations of diode sensing elements inaccordance with embodiments of the invention;

FIGS. 5, 6, 7, 8, and 9 are lumped equivalent circuits of the diodesensing element of the invention over various impinging frequencies;

FIG. 10 illustrates a thermocouple sensing element having distributedthermocouple film elements and the geometry thereof;

FIG. 11 is the lumped equivalent circuit of a probe with singlethermocouple elements relative to a total probe; and

FIG. 12 is a block diagram of an instrument embodying the features ofthe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The probe 10 shown in FIG. 1, serves to mount the sensors of theinvention and also modifies the signals generated therein fortransmission over a line 17 to a metering instrument. This is explainedin greater detail later in the specification. In essence, three sets ofsensing elements 11a, 11b, and 11c are supported at one end of probe 10along mutually orthogonal axes.

By proper selection of sensor elements, a particular probe provides thesensitivity response characteristics shown graphically in FIG. 2. Properhandling of the signals generated with this sensitivity, affords avariety of monitoring opportunities. It will be recalled that the lowfrequency sensitivity of the monitor described herein is determined bydiode sensors while the high frequency sensitivity is determined bythermocouple sensors. In FIG. 2, the low frequency region is shown toextend generally from 0 through 1.5 GHz and the high frequency regionextends generally from 1.5 GHz to 40 GHz and beyond.

Before considering in greater detail the specific characteristics of thevarious sensing elements, a visual appreciation of the elements andtheir interrelationship will be available from FIG. 3. This figure showsan array of series-connected thermocouple dipoles 18, making up the highfrequency sensors, axially aligned with a dipole antenna 19, whichserves as the input source for the low frequency diode sensor. Threesuch sets of sensing elements provide a complete unit for isotropicresponse of an ultra-broadband of frequencies.

By utilizing separate sensing elements for different portions of a broadfrequency range, one is able to select the sensing elementcharacteristics best suited to either the portion of the range ofinterest, or best suited to the measurement of interest within thatportion of the range. In addition, one is able to trim each sensingelement to match the type of response desired within each portion of thebroad range. It might be the designer's intent to produce constantsensor sensitivity, or response, across the entire range. It is alsopossible to tailor the response to predetermined criteria, asillustrated with the embodiment disclosed herein.

The low frequency diode sensor element is illustrated in FIGS. 4A and4B, and the lumped equivalent circuit for such a sensor is shown in FIG.5. The equivalent inductive and capacitive reactance of the dipolesegments is illustrated separately as L_(a1), L_(a2) and C_(a1), C_(a2),respectively. When considering the combined effect of such reactances,reference will be made to L_(a) and C_(a), respectively. Similarly, thevoltage induced in the dipole responsive to illuminating energy isillustrated by separate sources, e. The equivalent resistance of thedipole is illustrated as R_(a).

In one specific embodiment of the invention, beam lead Schotky diodeswere used to minimize connecting loops that might couple to the fieldand introduce erroneous signals. In addition, the size of the resistiveand capacitive components was minimized by use of chip components.Typical values of the components shown in conjunction with the diodesensing element were (with a total dipole length of 3.75 inches):

C₁ =12 pfds.

C₂ =200 pfds.

C_(a1) =0.1 pfds.

C_(a2) =0.2 pfds.

R₂ =330 ohms

R_(a) =800 ohms

L_(a1) +L_(a2) =0.036 mh

In the frequency range from 30 to 300 MHz, the equivalent circuitsimplifies to that shown in FIG. 8 wherein the dipole resistance R_(a)and dipole inductive reactance of L_(a) are negligible compared to thedipole capacitive reactance of C_(a). Within this range, reactance of C₂is selected to be very low, and resistance R₂ is selected to beextremely high relative to the reactance of shunting capacitor C₁. Thedipole capacitance C_(a) and the shunting capacitance C₁ act as acapacitive voltage divider having a uniform output with frequency overthis range, the capacitance C_(L) having been shunted across the diode Dto obtain the desired sensitivity.

Descending in frequency and commencing at 30 MHz, the shape of theresponse is principally controlled by the resistance R₂ in conjunctionwith the reactance of shunting capacitor C₁, and the dipole capacitanceC_(a). The equivalent circuit then reduces to the one shown in FIG. 7.The radio frequency voltage across the diode D varies at a 6 db peroctave rate as the frequency decreases to 3 MHz. The diode DC output,having a square law characteristic, provides an output proportional tothe square of the radio frequency voltage across the diode. The diodeoutput sensitivity therefore decreases as the square of frequency, f².

Descending still further in frequency, at approximately 3 MHz thereactance C₂ increases to the magnitude where it exceeds the resistanceR₂ and the equivalent circuit reduces to approximate the schematic ofFIG. 6. Hence, the circuit again performs as a capacitive divider havinga constant sensitivity with frequencies below 3 MHz.

Within the frequency range above 300 MHz and to 1.5 GHz, the dipolecharacteristics are selectively modified to achieve controlled roll-off.The dipole is constructed of conductive and resistive portions, asillustrated in FIG. 4A. The conductive portion connects a resistive filmportion to the terminals of the diode. The extent and location of theresistive film portion of the dipole is selected to achieve the desiredcharacteristics. When the frequency increases above 300 MHz, theresistance of this film predominates over the reactance of that portionof the antenna. If the entire dipole had been constructed of resistancematerial, the sensitivity would decrease at a 6 db per octave rate, or1/f². To approximate the desired 1/f, 3 db per octave rate, a portiononly of the dipole is made resistive, the resistivity in the presentcase being chosen to start roll-off at 300 MHz. That portion of theconductive dipole contributes a signal to the diode that is constantover the frequency range thereby achieving the desired reduction insensitivity to approximate 1/f. The diode operates in its square lawregion and the induced voltages are made approximately equal for bothconductive and resistive portions.

In the low to high frequency transition 300 MHz to 1.5 GHz range, theequivalent circuit of FIG. 5 reduces to that of FIG. 9, where theresistance R₂ is much greater than the shunting reactance of C₁ and thedipole reactance L_(a) is negligible.

Under these circumstances the diode output voltage V_(d) becomes:##EQU1##

The response of the diode sensor thus decreases as the frequencyincreases, but the addition of the output from the higher frequencysensor elements as described hereafter, will produce additionalcorrection. This technique can be modified further by using discreteresistances strategically placed along the dipole length. FIG. 4Billustrates this structure with a single resistive pad R_(P) on each legof the dipole.

The resistivity of the dipole and the concomitant decreasingsensitivity, prevents affecting the response in the high frequencyregion as the dipole approaches resonance. The values of capacitance andinductance for the sections of the dipole can be calculated from theaverage characteristic impedance for each portion of the dipole. Theequation for characteristic impedance Z₀ is: ##EQU2## where H₁ and H₂define the limits of each portion of the dipole, and a is the effectiveradius of the dipole. The equation can then be integrated over theportion of the antenna in question.

Using the expression of L_(a) and C_(a) :

    L.sub.a =Z.sub.0 (8f.sub.0).sup.-1

    C.sub.a +(π.sup.2 f.sub.0 Z.sub.0).sup.-1

The thermocouple sensing elements used for the high frequency region,are described and explained structurally in the inventor'saforementioned U.S. Pat. No. 3,641,439 and in his I.E.E.E. article. Theprobe may consist of three mutually perpendicular broadband probeelements as illustrated in FIG. 1. Broadband characteristics areobtained by distributing resistive thermocouple dipoles along the lengthof the high frequency sensing element at spacings that will permit noresonant length over the range of frequencies within which the probe isintended to operate. The spacing is less than one-quarter wavelength ofthe highest frequency to be measured. In effect, the high frequencysensing element may be viewed as a group of series-connected smallresistive dipoles or as a very low Q resonant circuit.

Each thermocouple dipole element and/or connected set provides a DCoutput signal that is proportional to the square of the electric fieldstrength tangential to the element. The elements are preferablythin-film thermocouples that provide true square-law outputs. The DCsignal is proportional to the power dissipated in the thermocoupleelements and indicates the average energy density in the volume in whichthe elements are contained. The summation of the DC signals from thethree orthogonal sensing elements provides a measure of the total energyor power density, independent of direction or polarization of the RFsignals.

A lumped equivalent schematic representation of a thermocouple sensingelement is shown in FIG. 11. L₃ and C₃ are the lumped equivalentinductance and capacitance of the element determined from the averagecharacteristic impedance Z₀.

    C.sub.3 =2(π.sup.2 f.sub.0 Z.sub.0).sup.-1

    L.sub.3 =Z.sub.0 (8f.sub.0).sup.-1

f₀ is the resonant frequency of a dipole of the same length as theelement. R₃ equals the total resistance of the probe element less R₄. C₄is the shunt capacitance across a small dipole and can be determinedfrom the geometry of FIG. 10 as ##EQU3## The DC output of the smallthermocouple dipole is proportional to the power dissipated in it.

The resistive dipoles are composed of thin films of overlappingdissimilar resistive films 30, 31, deposited upon a thin plasticsubstrate. The geometry creates alternate cold and hot junctions. Asshown in FIG. 10, the hot junctions are formed at the center 32 of thenarrow strips having relatively high resistance thereby allowing for thedissipation of energy and the resultant increases in temperature. Thewider sections 33, 34 have a low resistance and thus function as coldjunctions, the low resistance allowing little energy to be dissipatedwithin these sections. In addition, the broad area distributes theenergy and conducts heat rapidly into the substrate so that very littletemperature rise occurs. The resultant DC output voltage is directlyproportional to the energy dissipated in the resistive portion of thethermocouple.

The spacing "D" between the cold junctions is a small fraction of amillimeter. The close spacing minimizes zero drift due to ambienttemperature, since only a very small temperature gradient can occur dueto the variation in ambient temperature. Variation in the ambienttemperature will cause variation in sensitivity that is less than 0.05percent/°C., which will not degrade the basic accuracy of the instrumenteven over wide temperature ranges. The leads that carry DC outputs fromthe probe elements to the metering instrumentation are high-resistancefilms and present a high resistance near the probe elements resulting inlow current to minimize any interaction of the DC leads and the probeelements.

The total effect of the structure described is to approximate thecondition of the high frequency sensing elements being suspended inspace because the leads are transparent to the RF fields. The extremelylight coupling into the field, results in very little perturbation ofthe RF field being measured due to scattering phenomena. A break pointof the frequency sensitivity curve is provided at 1.5 GHz at the lowfrequency end and above 40 GHz at the high frequency end. This yieldsextremely flat frequency response from 1.5 GHz, while below 1.5 GHz theresponse decreases at a 6 db/octave rate.

To minimize cross coupling between the resistive high frequencythermocouple dipole and the low frequency diode connected dipole, eachelement pair is placed on the same axis, with some spacing between theadjacent ends. FIG. 3 shows the spacial relationship.

With an appreciation of the structure and arrangement of the low andhigh frequency sensor elements of the invention, it will be understoodthat monitoring instrumentation of various forms can be adapted tocooperate with a properly designed probe. FIG. 12 is a block diagram ofone such form of instrumentation.

DC signals from a housing 11 containing suitably mounted high frequencythermocouples and low frequency diode elements, are delivered to apre-amplifier in the handle of the probe 10, over a high resistivitytransmission line 12. A first two-wire DC transmission line 13 is usedfor the three high frequency elements which are connected in series, aspreviously explained. A second two-wire DC transmission line 14 is usedfor the three diode elements which are connected in parallel, as alsopreviously explained. The leads are held rigidly in place to preventcable modulation. A high resistivity film is advantageously applied overthe probe to provide shielding from static charges. The pre-amplifiersutilize balanced instrumentation amplifiers and the resistivetransmission line leads are matched to further reduce common mode andstatic charge induced signals.

The pre-amplifier contains two sections. One section 15 conditions thediode signal and provides temperature compensation and such linearitycorrection as may be required at the upper extreme of the operatingpower density range. A gain setting control may be provided forcalibration. The conditioning amplifiers 16 for the high frequencythin-film thermocouple sensors need provide for calibration only,inasmuch as temperature compensation and linearity correction are notnecessary. The signals from the pre-amplifiers are transmitted over aconductive cable 17 to a suitable metering instrument.

Within the metering instrument shown in FIG. 12, the two signals fromthe probe are first combined in a summing amplifier 20. The resultantsignal is further processed by a maximum hold amplifier 21. The outputof this latter circuit feeds an amplifier 22 which in turn drives aD'Arsonval meter movement 24.

A particular ultra-broadband radiation monitor has been described. It isrecognized that modifications will be apparent to those skilled in theart. All modifications coming within the teachings of this disclosureare intended to be covered by the following claims.

What I claim is:
 1. An electric field sensitive probe comprising a diodeconnected to a dipole antenna, shunt capacitive means connected inparallel with said diode, and a circuit comprising series-connectedresistive means and capacitive means in parallel with said diode, saiddipole having conductive portions connected to said diode and resistiveportions remote from the connections with said diode, the impedance ofsaid components being selected to establish a voltage across said diodethat is substantially independent of frequency over a particular range,that decreases substantially proportional to the square of the frequencybelow said range, and that decreases substantially proportional to thefrequency above said range.
 2. An electric field sensitive probe asdefined in claim 1, wherein the impedance of said components is furtherselected to establish a frequency at which the voltage across said diodereaches a predetermined level following said decrease below said range,after which said level is maintained substantially independent offrequency.
 3. An electric field sensitive probe as defined in claim 1,wherein said shunt capacitive means and the capacitance of said dipoleeffectively function as a capacitive voltage divider throughout saidparticular range.
 4. An electric field sensitive probe as defined inclaim 3, wherein said resistive means, said shunt capacitive means, andthe capacitance of said dipole control the radio frequency voltageacross said diode to decrease at a 6 db per octave rate as the frequencydecreases below said particular range.
 5. An electric field sensitiveprobe as defined in claim 4, wherein the resistance of the dipolepredominates over the capacitive reactance of the dipole as thefrequency increases above said particular range, and wherein theimpedance of said resistive means is substantially greater than thereactance of said shunt capacitor above said particular range.
 6. Anelectric field sensitive probe as defined in claim 3, wherein at leastone discrete resistive film segment is positioned along each leg of saiddipole.